In the past, servo control systems have been forced into unstable oscillations when the mechanical device driven by the system has been subjected to some form of shock, for example, a rapid reversal of drive direction. This shock induces vibrations in the mechanical device at a frequency peculiar to that device. The frequency of the vibrations may be termed the natural resonating frequency of the device. The resonating frequency may be induced into the electronics of the servo system thereby producing a control signal with a frequency component corresponding to that of the natural resonating frequency. The introduction of the natural resonating frequency into the control signal may be considered as positive feedback which may result in the mechanical device vibrating uncontrollably.
One approach to eliminate the adverse effects of the resonant frequency has been to incorporate a twin-T or parallel-T notch filter with the notch frequency adjusted to the resonant frequency of the mechanical device. This method is outlined in Canadian Pat. No. 822,306 issued Sept. 2, 1969 and assigned to Westinghouse Canada Limited. The aforementioned Canadian Patent disclosed a circuit comprised of two "T" configurations. The trunk section of the first "T" is formed by a resistor extending from a ground connection to a connection between a pair of capacitors on either side of the trunk and the capacitors form the horizontal segment of the "T". The trunk section of the second "T" is formed by a capacitor connected from the ground connection to a point between a pair of resistors on either side of the trunk and the resistors form the horizontal segment of the second "T". The end terminals of the pair of resistors are further connected in parallel with the end terminals of the pair of capacitors. While the circuit described in the aforementioned Canadian Patent results in stable operation of the servo system, it has the disadvantage that once the capacitors and resistors are fixed, the centre frequency, the bandwidth, and the attenuation factor of the notch filter can not be varied to minimize the phase delay of the system. The delay in the response of the system may result in large amounts of overshoot being present in the motion response of the mechanical device due to a change of the control signal.